The present invention relates to a load cell comprising a symmetrical mounting arrangement. More particularly, the invention relates to a load cell comprising a symmetrical mounting arrangement capable of mounting effect amelioration. The invention further relates to a weigh scale system comprising a load cell comprising a symmetrical mounting arrangement.
Typically load cells are mounted to a support structure in a scale system. The load cell is mounted to the support structure at its bottom or to one or more of its sides at one end of the load cell in a scale system. The loading fixture is mounted at the top or to one or more sides of the opposite end of the load cell. The load cell is made stiff at these ends to reduce distortion from so-called mounting and loading effects. A load cell is subject to “shear” when subjected to load changes such as is the case when loading the loading fixture of the load cell. This results in the so-called “loading effect”. Shear results from the spring-like behaviour of load cells and the necessity of the load cell to deform in order to measure an applied load. As such, a load cell of finite stiffness must have spring-like behaviour, exhibiting deflection based on spring constants. A distorted data pattern can be the result of uncorrelated shear caused by forces other than the desired load, leading to inaccuracies in measurement data from the load cell. Previous attempts to control the loading effect have involved stiffening to reduce the uncorrelated shear of the load cell. Although performance is enhanced by the presence of stiffening, the solution involves increased material in the weigh scale and an increase in the cost of manufacture.
The so-called “mounting effect” can be seen as a result of mounting the load cell on the adjacent support structure and/or of mounting the loading fixture on the load cell. Bolts attaching the load cell to the attachments distort the load cell and cause output changes that are undetermined and that change with changes in load and temperature and even time.
These loading and mounting effects can be partially compensated for when the load cell is calibrated to make the load cell as accurate as possible in that configuration. However, the compensation is limited to lower accuracy load cells with poorer resolution, since these effects are undetermined and caused by unstable frictional joints.
The mounting and loading effects are only reduced by using a relatively stiff-ended load cell and through calibration of the scale system incorporating the load cell. As a result, the performance of the load cell is compromised. Particularly, for load cells wherein a lower resolution and accuracy is required, for example when between 500 to 10,000 unit divisions is required, controlling the stiffness of the load cell ends themselves may be deemed to be adequate. However, the desire to have higher resolution and accuracy, for example when between 25,000 and 100,000 unit divisions are required, as is the case for example in part counters and pharmaceutical scales, requires an improved solution than the reduction of the loading and mounting effects seen as a result of controlling the stiffness of the ends of the load cell.